In here I will explain how you can defeat Sephiroth after reaching the final door on the Castle That Never Was.

Preparations for the fight.

One thing you should know is that after you cross the door, you can't go back, which means you have to clear the game, MAKE SURE YOU SAVED THE GAME BEFORE YOU CROSS IT, BECAUSE AFTER YOU PASS IT YOU WILL ONLY BE ABLE TO GET BACK TO THE LAST SAVE YOU DID, SINCE THE GAME DOES NOT SAVE AFTER YOU CLEAR THE GAME, in fact, after you clear the game, the ending scene and your game status appears and all, you will be FORCED to reset the console, because the game will literally stop at the status screen, only the music playing normally and repeatly.

When you reach the part the final door appears, all the secrets you haven't unlocked during the game will show up, telling what to do to get the rewards of doing those achievements, this is the signal you can go after Sephiroth.

One thing you HAVE to keep in mind is to save ALL the elixirs (or megalixirs) as possible until the end of the game, they WILL help you defeat Sephiroth, this is something CRUCIAL to keep in mind, so NEVER equip/use them!

During the game you will collect MANY power up items, which increases a character's strength, magic or defense, since MANY times Sora have to fight alone, ALWAYS use those items on him, since if at anytime your HP drops to 0, you get a game over, got a power up item? To Sora it goes.

MAKE SURE YOU COME WITH IMPORTANT SKILLS EQUIPPED, You will want to be able to see his HP (which is HUGE), be able to guard and counter-attack, aerial counter-attack (when you are launched into the air), Trinity, Trinity time extent, Keep 1 HP if being hit with a combo, keep 1 HP when a massive hit is received, Double jump (which can be obtained by training your forms), the fast run skill (which can be obtained by training your Wisdom form) and many other useful things to help you fight Sephiroth, Lucky Lucky is a skill you DON'T WANT to equip, since Sephiroth doesn't drop any items, you probably get the idea, about the form's skills, check the form statuses for info about how to train them, you can train your Valor form by delivering attacks, you can train the Wisdom form by defeating heartless only, Master can be trained by obtaining the yellow orbs which increases  your drive gauge when on that form, there is a secret form in this game, and to train that form you need to defeat Nobodies on that form, but it is not really required, so don't bother with this.

USE SHORTCUTS! They WILL come in handy in a tight situation, specially if you are in danger condition (the status starts flashing red with a siren-like sound playing), you will want to equip essential things like Curaga, Reflega, Elixir and the Megalixir, being able to quickly use those will greatly increase your chance of defeating Sephiroth.

EQUIP AS MANY ELIXIRS AS POSSIBLE! That's why I asked you to save them during the whole game, they are quite hard to come, so try as much as you can to save them!

Fighting Sephiroth!

After the preparations for the fight are done, head to Hollow Bastion, remember the part you had to fight OVER 9000! 1000 heartless? Which you passed through a crystal cave and in the end of the path you encountered with Saíx? That is where Sephiroth is, head there.

When you reach there, Sephiroth would have not noticed you yet, you would have to approach him and use the Reaction command "Approach" to trigger the cut scene before the fight.

When the battle starts, you notice that Donald and Goofy have left you (Traitors!)! Good thing you used all the power up items on yourself, right??? But sadly you will not be able to use the Drive forms, so you will have to defeat him with your skills!

When the battle starts, the FIRST thing you want to do ALWAYS, is to QUICKLY press Triangle! If you don't do it, Sephiroth you literally beat you completely! HIS ATTACKS ARE INSANE! Trust me, I have fought and defeated him, and I know how strong he is, if are caught by this initial combo, you will either killed in an INSTANT (if you don't have the 1 HP left in a combo skill equipped) or leave you with ONLY 1 HP left, it is very important to always do this, but don't worry, this attack is not invisible, you will know when he is going to do that when the sky starts getting very dark.

When he is on the floor (his sword over his shoulder pointing at you) DON'T ATTACK HIM! Move close to him let him start another combo! But make sure you guard! Otherwise this combo WILL also leave you with only 1 HP left or kill you without any chance of defense! After guarding, COUNTER ATTACK! And do your combo! Attack until your finishing move! After the combo ends, you will notice that Sephiroth will vanish, and suddenly he will show up in front of you to make a uppercut attack which will launch you VERY HIGH, LET HIM HIT YOU, this attack don't take too much HP compared to the other attacks (His normal attacks can LITERALLY take about 1/3 of your TOTAL HP!) Once he have hit you and launched you High, THIS IS YOUR CHANCE TO DO A POWERFUL COMBO AGAINST HIM! Counter attack and do you normal combo again and USE YOUR TRINITY! Not only you would have damaged him with your normal combo, but you would add damage with your trinity! And to make things better, you are LITERALLY invincible when on Trinity! So make good use of it! After your trinity ends, the MP gauge will start to fill again (you will know when the MP gauge is filled when the red MP gauge vanishes and the blue one starts appearing again), during this time, try to keep the same strategy, defending his normal and the dark sky combo and letting him hit you when he vanishes to try to launch you high, and counter attacking in the air, DON'T HESITATE to use either Curaga (if you have your MP gauge filled or at least 1 MP left) or an elixir if you are in danger!

After you beat up Sephiroth a bit, you will notice his attack patterns change, he will fly around the place and eventually try to hit, try to guard on those moments, and TRY AS MUCH AS POSSIBLE to stay away from him when he is on the floor! Because if you are close he will make huge fire pillars appear around him! And this attack with attract you to it! Run as much as possible and use the Run skill you got by training the Wisdom form!That way you will be able to escape from the Fire pillars! And you will notice that sometimes he will make dark spheres appear around you, try to use Reflega to make them go away, specially when the spheres starts going to you! One thing you will have to watch at ALL times, is when Sephiroth starts flying upwards slowly like an angel, THIS ATTACK IS @!%#&*! DANGEROUS! Not only he will decrease your HP to 1, but he will ALSO drain your MP for it to start recharging in a blink of an eye! If you have your MP gauge on blue, USE TRINITY! A single attack on him can stop it! If you don't have MP, let him do the attack, then ASAP use an elixir! You with both recover your HP completely, but your MP gauge will get active again!

If you keep those ideas in mind, you will eventually see yourself defeating Sephiroth! If you managed to defeat him, congratulations! He is the STRONGEST ENEMY ON THE WHOLE GAME! If you manage to defeat him, you will increase your DRIVE GAUGE by 1! And if you do what Sephiroth asks, you will get A NEW KEYBLADE from Tifa, I hope you enjoy the scene ^-^

If you liked this or would like to give a suggestion or anything, feel free to leave a message here! It have been quite a while since I last played Kingdom Hearts, so some things I may have forgotten, but I left here the most important tips to defeat him.

This was Seishiro Leonhart explaining to you how to defeat Sephiroth like a boss!

...this looks... FANTASTIC! KH players like me will surely appreciate this.